1. Field of the Invention
The present invention relates to an electron gun for a color cathode ray tube.
2. Discussion of the Related Art
The electron gun for a color cathode ray tube is means for displaying desired information by emitting three electron beams and focusing them on a screen, to cause respective electron beams to luminesce R, G, B fluorescent materials to form a pixel. Main functions of the electron gun are formation of an object point by the electron beams and focusing of the electron beams, which are main factors that determine a luminance and resolution of the color cathode ray tube. The most important factor that determines the luminance of the color cathode ray tube is a beam current, and the most important factor that determines the resolution is a size of a beam spot. Because the size of a beam spot becomes the larger as the beam current becomes the greater, realization of a high luminance and a high resolution on the same time has been impracticable, and requirements for high degrees of control technologies for the electron gun, deflection means, screen and circuit result to high cost of parts. In the meantime, CPT (color picture tube) mostly for use in displaying moving pictures, such as television and video watching and CDT (color data tube) mostly for use in CAD and displaying character information are manufactured differently. In most cases, the CPT is provided to display a moving picture and should have a high luminance aid contrast, and the CDT is provided to display character and graphic information and should have a high resolution.
The structure and operation of a background art electron gun for a color cathode ray tube will be explained with reference to FIGS. 1 to 7. FIG. 1 illustrates a horizontal section of the background art electron gun for a color cathode ray tube, and FIG. 2 illustrates a vertical section of the background art electron gun for a color cathode ray tube.
Referring to FIG. 1 and 2, the background art electron gun is provided with R, G, B cathodes 1, 2 and 3, a first electrode 4, a second electrode 5, a focus electrode 6, anode 7 and a shield cup 8, with three apertures in a horizontal direction for pass-through of the electron beams in each of the electrodes. The R, G, B cathodes 1, 2 and 3 are provided for emitting electron beams, the first electrode 4 is provided for controlling amounts of the electron beam emissions, the second electrode 5 is provided for accelerating the electron beams, and the focusing electrode 6 and the anode 7 converge the electron beams. Each of the electrodes are applied of an appropriate level of voltage for conducting a required operation. That is, each of the R, G, B cathodes 1, 2 and 3 is applied of a few to a few tens of a voltage, the first electrode 4 is applied of zero voltage, the second electrode 5 is applied of a few hundreds of a voltage, the focusing electrode 6 is applied of a few thousands of a voltage, and the anode 7 is applied of a few tens of a thousand voltage. Thus, upon application of different voltages to respective electrodes, paths of the electron beams are formed as shown in FIG. 4 along which the electron beams proceed. That is, an electric field by the second electrode 5, being an accelerating electrode, influences up to the cathode 2 so that electrons emitted from the cathode 2 is extracted from the cathode 2. In this instance, the first electrode 4, applied of a voltage at a level lower than the cathode 2 and the second electrode 5, suppresses the electron beam extraction to control an amount of the electron beam extraction by means of a voltage difference between them. In general, the voltage to the first electrode 4 is fixed while the voltage to the cathode 2 is varied, for controlling the amount of the electron beam emission.
In the meantime, an electron beam from a surface of the cathode is crossed through its beam axis by voltage distributions of the cathode, the first electrode and the second electrode. That is, as shown in FIG. 3, an electron beam crossing 100 is occurred by the electric field. After occurrence of the crossing 100, the electron beam 101 is made to advance toward the screen by electric fields becoming higher from the second electrode 5 to the focusing electrode 6, during which process, the electron beam 101 is subjected to primary convergence by a weak electric field formed between the second electrode 5 and the focusing electrode 6, and subjected to secondary convergence by a strong electric field formed between the focusing electrode 6 and the anode 8, so as to be focused on a point of the screen.
FIG. 4 illustrates an equivalent optical model of a beam path formed by the electric fields in the electron gun. A lens formed by the electric field between the second electrode 5 and the focusing electrode is called a pre-focusing lens 10, and a lens formed by the electric field between the focusing electrode 6 and the anode 7 is called a main lens 12. Being a beam 102 subjected to primary convergence by the pre-focusing lens 10, the beam incident to the main lens 12 is different from the beam 101 crossed before the pre-focusing. The beam 102 before incident to the main lens 12 is drawn only to show outermost beam contours, but the beam 102 has numerous fluxes of electron beams as shown in FIG. 3, actually. Reverse direction extension lines of the beam 102 travel paths form a virtual crossing 110 on a cathode 2 side. This virtual crossing point is an object point 111 of the main lens 12, a size of the electron beam at the point is an object radius Rx, and a distance from the object point 111 to the main lens 12 is an object distance p. On the other hand, the electron beam passed through the main lens 12 is focused onto the screen, which focused point is called an image point 112, and a distance from the main lens 12 to the image point is an image distance q, and a size of the electron beam at the image point is a spot size Dt. Eventually, an object point 111 with a radius Rx and a distance p is formed into an image point 112 with a spot size Dt on the screen. In the meantime, inclusive of the prefocusing lens 10 which forms the current beam and the object point incident to the main lens 12, there is a triode with the cathodes, the first electrode, the second electrode, and up to the apertures in the focusing electrode on the cathode side. The size Dt of the electron beam spot which is a main factor for determining a resolution of a color cathode ray tube may be expressed as an equation 1 shown below. EQU Dt=M.times.Dx,
where, M is a magnifying power of the main lens determined by a shape of the lens and a voltage applied thereto, and Dx, being 2Rx, is an object diameter. As can be known from the equation 1, the object diameter is a main factor that determines the resolution. In the electron gun, the object diameter becomes the greater as a current is the greater, that is dependent on diameters of the apertures of the first and second electrodes.
FIG. 5 illustrates a vertical section of a triode in an electron gun for a CPT which has a high luminance and a low resolution, and FIG. 6 illustrates a vertical section of a triode in an electron gun for a CDT which has a low luminance and a high resolution.
Referring to FIGS. 5 and 6, the structure and operation of the two electron guns are the same with the explanations given, they have differences in their aperture diameters of the first and second electrodes. That is, a Rp, the aperture diameter either of the first and second electrodes 4 and 5 of the CPT electron gun shown in FIG. 5 is greater than a Rd, the aperture diameter either of the first and second electrodes 4-1 and 5-1 of the CDT electron gun shown in FIG. 6, for using a large current range without any excessive increase of a cathode load required for providing a high luminance in the case of the CPT, which however means that a high resolution can not be obtained due to increased object diameter. On the contrary, in the case of a CDT which requires a high resolution, i.e., a small object diameter, the aperture diameters Rd of the first and second electrodes should be small, resulting in a reduced beam current. It can be explained in more detail as follows. The load on a cathode is an amount of beam current per a unit beam emission area of the cathode, i.e., a beam current concentration. Therefore, it is required to increase the emission area for increasing the beam current. And, a maximum beam emission area is a maximum area on the cathode to which the voltage of the second electrode can influence, with a maximum emission radius of Rp in the case of FIG. 5 and Rd in the case of FIG. 6. FIG. 7 illustrates a graph showing current range vs. spot size in background art electron guns for CPT and CDT. With reference to FIG. 7, it can be known that the CPT has a greater spot size and uses up to a high current range and the CDT has a smaller spot size and uses a low current range.
The background art electron gun for a color cathode ray tube has the following problems.
An electron gun for the CPT can not conduct a CDT mode, and vice versa. Accordingly, the background art electron guns are not adaptive to a multimedia environment of which importance increases gradually in which they should be used interchangeably, putting a limitation on a multipurpose use of the electron guns.